The objectives of this research proposal are to define the cellular and molecular actions of the hormonally active form of vitamin D, namely 1,25(OH)2D3, as its acts in its principal target organ the intestine to stimulte intestinal calcium transport (ICA) and to induce via genomic mechanisms the appearance of a 28K calcium binding protein (CaBP). The proposal builds on our recent successful cloning of the mRNA for the CaBP as well as on our observation of a "very rapid" (within 12 minutes) changes in ICA; both events are governed by 1,25(OH)2D3. The program includes the following: (a) Application of molecular biology/recombinant DNA techniques to the CaBP to define its amino acid sequence, its gene organization, to construct a SV-40-CaBP transvection vector for GH4-cells, and using cDNA probes to define changes in its mRNA levels; (b) Definition of the biodynamics of CaBP in the inestinal epithelial cell using a photoaffinity probe and by quantitation of its rates of synthesis and degradation, to ascertain whether CaBP functions dynamically in events associated with ICA; (c) Evaluation of the interaction of the receptor for 1,25(OH)2D3 with selected regions of the 28I-CaBP gene and determination of how vitamin D status may program chromatin organization to make selected genes in selected target organs available for hormonal regulation; specification of the ligand prefernces for the receptor; and (d) Elucidation of the cell biological and biochemical details of the "very rapid" ICA response to 1,25(OH)2D3. Collectively these results will define the time course spectrum of action to 1,25(OH)2D3 in the intestine as relates both to genomic action and ICA and provide insight into the cellular interactions of the CaBP. The implications of the results will extend in a basic sense to all aspects of the far-reaching vitamin D endocrine system and in a clinical sense to the many disease states which are implicated in this framework, e.g. osteoporosis, sarcoidosis, renal osteodystrophy and vitamin D resistant rickets.